Communications systems that utilize non-stationary users or relay sites frequently use directional antennas to direct radio signals between stations. Communications systems are designed to be as efficient as possible, both in terms of spectral efficiency and power efficiency. Moving stations require tracking systems to assist in pointing radio signal beams. Tracking systems ensure that beams widths can be kept as narrow as possible. Narrow beam widths reduce the amount of power necessary for effective communication between stations, prevent unwanted parties from potentially receiving signals, and prevent overlap of signals onto spatially adjacent receivers, which is important to minimize interference and for regulatory compliance.
Directional antennas require a high degree of pointing accuracy to maintain adequate power in the communication link and to minimize interference with neighboring receptors. Tracking systems maintain this pointing accuracy by correcting for errors such as inertial reference drift or target movement. Tracking systems work by inscribing modulation on a signal received from the tracked signal source, for example a satellite. The modulation is dependent on the motion, or scan path, of the directional antenna. The attendant modulation is used to deduce relative pointing position. Prior art tracking systems generally require highly directional antennas and several degrees of separation between satellites to ensure tracking signals only arise from the desired source. In practice however, antennas are often small aperture antennas and station separation is minimal, creating the possibility of impairment to the tracking signal from those nearby stations. These impairments change the nature of the tracking signal, corrupting the tracking signal and degrading the accuracy of the tracking capability. The resulting mis-pointing reduces the effective data capacity of data links between stations.
Several systems and methods were developed to improve the tracking algorithms of directional communications systems and correct for impairments to tracking signals by adjacent satellites. U.S. patent application Ser. No. 12/277,192 filed Nov. 24, 2008, entitled “Burst Optimized Tracking Algorithm” includes a system and method for optimizing tracking algorithms of directional communications systems for enhancing data communications between sending and receiving parties. U.S. patent application Ser. No. 12/371,866 filed Feb. 16, 2009, entitled “Robust VSAT Tracking Algorithm” includes a system and method to reduce the effect of signal impairments from interfering sources by measuring the mathematical morphologic features of the detected tracking signal and using those features to normalize the pointing of the antenna against interference induced pointing error. Improvements to the accuracy of the scan path actually traversed will improve the function of the above tracking methods.
Small aperture antennas are often used on mobile platforms like aircraft, because of size and weight considerations. Small aperture antennas can be directionally agile antennas capable of being steered towards a tracked satellite. Phased array antennas are one kind of directionally agile antenna. Because phased array antennas are steered electronically, it is difficult to accurately detect deviations in the actual scan path taken in relation to the intended scan path. When the scanning is performed without any feedback, it is described as an open loop system. Some small aperture antennas use inertial pointing based on inertial reference units instead of tracking systems to point at the desired satellite and to stabilize the antenna orientation. But inertial reference units are susceptible to drift over time and operate as an open loop system without feedback for pointing accuracy. Because of this lack of feedback, mechanical movement accuracy of antennas degrades due to mechanical wear and perturbation from external sources, such as vibration, misbalancing, residual torques, or bearing friction.
Accurate scan paths improve feature accuracy for received signal patterns.